feat(core): add cursor persistence module for session-based timestamps

Introduces a lightweight file-based cursor system for persisting
per-user timestamps across CLI invocations. This enables "since last
check" semantics where `lore me` can track what the user has seen.

Key design decisions:
- Per-user cursor files: ~/.local/share/lore/me_cursor_<username>.json
- Atomic writes via temp-file + rename pattern (crash-safe)
- Graceful degradation: missing/corrupt files return None
- Username sanitization: non-safe chars replaced with underscore

The cursor module provides three operations:
- read_cursor(username) -> Option<i64>: read last-check timestamp
- write_cursor(username, timestamp_ms): atomically persist timestamp  
- reset_cursor(username): delete cursor file (no-op if missing)

Tests cover: missing file, roundtrip, per-user isolation, reset
isolation, JSON validity after overwrites, corrupt file handling.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/paths.rs

@@ -40,6 +40,20 @@ pub fn get_log_dir(config_override: Option<&str>) -> PathBuf {
     get_data_dir().join("logs")
 }
 
+pub fn get_cursor_path(username: &str) -> PathBuf {
+    let safe_username: String = username
+        .chars()
+        .map(|ch| {
+            if ch.is_ascii_alphanumeric() || matches!(ch, '_' | '-' | '.') {
+                ch
+            } else {
+                '_'
+            }
+        })
+        .collect();
+    get_data_dir().join(format!("me_cursor_{safe_username}.json"))
+}
+
 pub fn get_backup_dir(config_override: Option<&str>) -> PathBuf {
     if let Some(path) = config_override {
         return PathBuf::from(path);